1. Technical Field
The present invention relates in general to the lubrication of disk drives and, in particular, to an improved system, method, and apparatus for a disk drive equipped with special purpose additives that inhibit the passage of oil across an oil-air interface and enable the use of the lower viscosity oil in the disk drive.
2. Description of the Related Art
In the prior art, significant oil loss is observed from server class fluid bearing disk drive motors during accelerated life tests at elevated temperature. The pathway for oil loss from the upper part of a bearing in one design type is schematically illustrated in FIG. 1. FIG. 1 depicts a sectional view of an upper motor bearing 11 showing the oil 13, the pathway 15 for oil loss through a seal gap 17, and the seal 19 that slows down the rate at which oil leaves the bearing cavity 21. Sealing the motor bearing 11 decreases the oil loss, but oil is still emitted through the required air gap 17 in the seal 19.
For example, the oil loss from a bearing during accelerated motor reliability testing is shown in FIG. 2. FIG. 2 illustrates the oil remaining in a bearing as a function of time during continuous running at elevated temperature. The bearing was tested with a seal (upper line 25) and without a seal (lower line 27).
As shown in FIG. 3, the oil 13 exits the bearing 11 by first passing across the oil-air interface 31 into the bearing cavity 21, possibly via oil-air interface instability. The oil then convects and diffuses (e.g., indicted by arrows 15) as oil mist 33 and/or oil vapor 35 out of the interstitial region of the bearing cavity 21 through the seal gap 17. As demonstrated in FIG. 2, the oil loss from the cavity can be reduced with an improved seal gap configuration. Oil loss also can be decreased through the use of a less volatile oil. However, that option is a less desirable solution since an oil with lower volatility has a higher viscosity that can result in a 30% increase in power consumption and difficulty in cold starts. Thus, an improved solution would be desirable.
It is known that hydrocarbon monolayers inhibit vaporization of water by as much as 40 or 50% in applications involving cooling water in evaporative towers. See, e.g., U.S. Pat. Nos. 4,099,915 and 4,147,658. Hydrocarbon surfactants cannot readily form a low surface tension monolayer on the surface of another hydrocarbon, so these methods have not been successfully applied to suppress oil evaporation. Since evaporation of volatile hydrocarbons such as solvents and gasoline is a recognized problem, low permeability aqueous foam has been used to cover those types of surfaces. See, e.g., U.S. Pat. Nos. 5,434,192 and 5,296,164. However, a foam will not work in the fluid bearing motor of a disk drive because of the very small annular space limitations and because foams are not stable for the long life required of magnetic storage devices. U.S. Pat. No. 5,935,276 discloses another solution that uses a soluble polymer film to inhibit the evaporation of solvents or gasoline. Although polymers increase the viscosity of the fluid bearing oil, it does not provide enough suppression of evaporation.
Thus, none of the known solutions for suppressing the evaporation of fuel or oil are sufficient to provide an integral evaporation barrier in the presence of interfacial shear flow that is present on the oil meniscus in a disk drive fluid bearing spindle motor. In contrast, the invention disclosed herein teaches that novel fluorohydrocarbon surfactants form a layer on oil that mimics the evaporation suppression of hydrocarbon surfactants on water, and remains intact in the presence of surface shear flow of a disk drive motor.